1. Field of the Invention
The present invention relates to a nonvolatile semiconductor device having a parallel write and read function.
2. Description of the Related Art
In a nonvolatile semiconductor memory device, an electrical write operation can be carried out after the device is mounted on a printed circuit board. One typical example of such a device is a flash memory which has an advantage in that data storage is possible without a backup battery and which is highly integrated.
In such a flash memory, when an erase operation is performed upon a predetermined sector of memory cells or all of the memory cells simultaneously, the threshold voltages of the erased memory cells are fluctuated. In order to avoid such fluctuation, before a flash erase operation, data such as "1" is sequentially written into each of the memory cells, and thereafter, data is sequentially read from each of the memory cells and is compared with expected data which is, in this case, "1", thus requiring a considerable time for a flash erase operation. This will be explained later in detail.